The present invention relates to an aircraft tracking system, and more particularly, to a method of utilizing information from a bottom antenna of an aircraft to determine bearing of other aircraft.
The increased demands placed o the aircraft flight deck as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring of aircraft traffic in a vicinity of an aircraft that includes automatic identification of potential threats to the monitoring aircraft. As a result, aircraft can have transponders associated therewith that, in response to appropriate electromagnetic interrogation signals provide responding electromagnetic signals that can include information with respect to the range, altitude, and bearing of the interrogated aircraft. Certain traffic control system transponders, e.g., the mode-S system, include target identification as part of the information imposed on the responding electromagnetic signals. For these mode-S systems, the identification of the path or track of the responding aircraft is relatively simple, involving time dependent positions and altitudes of an identified aircraft. Similarly, extrapolations or extensions of aircraft tracks can be relatively simple.
In systems (such as the Air Traffic Control Radar Beacon System, ATCRBS) which do not include unique aircraft identification information, the determination of the aircraft tracks is more complicated. The information obtained by periodic interrogation of a multiplicity of unidentified targets, with associated range, altitude and bearing information being provided or determined as a result of the interrogation, can be subjected to well known algorithms to provide a target aircraft track. Once the track is identified, then the extension thereof can be computed to determine if the target aircraft is in the immediate vicinity and poses a threat to the monitoring aircraft.
Early Traffic Alert and Collision Avoidance Systems (TCAS) generally included a bottom omni-directional antenna, but usually did not include bottom directional antennas. In addition, however, many aircraft do include having a top directional antenna. Bottom directional antennas are being added to aircraft, and when the monitoring aircraft transmits an interrogation signal, a response can be received on both the top and bottom antennas. Although there is a "cone of silence" on the bottom antenna due to the antenna pattern, responses from the bottom directional antenna provide additional bearing coverage as a result of antenna elevation pattern and antenna location.
The operation of aircraft collision avoidance apparatus is governed by the Radio Technical Commission for Aeronautics (RTCA) Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System (TCAS) manual document number D0185. The MOPS document implies that all the bearing responses received from the top or bottom antennas are utilized equally. The preferred embodiment of the present invention categorizes and prioritizes the responses from the respective antennas, and utilizes the bearing measurements by priority to obtain the best bearing measurement possible.
The method of the present invention utilizes information received on the bottom antenna to improve the bearing reliability of the target aircraft by making use of the information from the bottom directional antenna. However, since the top antenna has more reliable information (the top antenna generally transmitting with more power, is subjected to fewer obstructions, and is less prone to multi-path interference, i.e., (false tracks), it is desirable to utilize the information received from the top antenna first rather than the bottom antenna. Thus, the method of the present invention includes a technique in which the bearing measurement is categorized and prioritized depending on whether the information utilized to generate the bearing measurement is received from the top antenna or the bottom antenna. It is still desirable to use the bottom antenna for bearing measurement to increase the bearing track coverage. The method of the present invention uses the best bearing measurement available in determining how to process the replies received from the antennas for track initiation, track extension, and track elimination. Overall, the method of the present invention results in an improved bearing measurement for the targets being tracked.